1. Field of the Invention
The present invention relates to a method for the synthesis of sulfo-N-hydroxy succinimide salts, novel reduced-impurity or impurity-free salts, and intermediate succinic diester sulfonate salts.
2. Background of the Art
Sulfo-N-hydroxy succinimides (often referred to as "Sulfo-NHS" or "S-NHS"), including the acid and salt counterparts, have a wide range of utility in a number of broad commercial areas, including but not limited to reagents for the manufacture of biotinylation reagents, oil well drilling agents, chemical and biological assay reagents, crosslinking agents for organic biological systems or polymer systems, side chain modifying agents, solubilizing agents, reactants, markers, and the like. The class may be generally represented by the formula below, representing the central nucleus of sulfo-N-hydroxysuccinimides: ##STR1## Wherein X.sup.+ is a cation and R is H or an organic group.
The compounds may also be described by the formula below, representing the central nucleus: ##STR2## Wherein X.sup.+ is a cation and R is H or an organic group, or especially any organic group formed from a compound R--OH wherein R--OH is an acid, and the OH (hydroxyl) which, when symbolically extracted leaves the group R-- to form an ester in the compound. Examples of preferred R--OH compounds are acetic acid, LC-biotin suberic acid, biotin, suberic acid, 4-N-maleimidomethyl!-cyclohexane-1-carboxylic acid, and the like. The 2- and 3- positions on the central nucleus may also be substituted. The cation may be H.sup.+, monovalent cations (such as H.sup.+, Na.sup.+, Li.sup.+, K.sup.+, NH.sub.4.sup.+, C.sub.s.sup.+, other inorganic cations, organic cations, etc.), or polyvalent cations (including divalent cations) in which the remaining charge is satisfied by other anions (e.g., halides, nitrates, sulfates, phosphates, etc.) or forms a bis- or tris- configuration with other Sulfo-NHS anions. This class of compounds is relatively expensive, mainly because of the expensive synthetic procedures which must be taken to obtain the product. Existing synthetic procedures must not only use a large number of reagents and involve a large number of synthetic steps, but the procedures involve the use of large volumes of solvents and different solvents which must be stripped after various steps as well as at the end of the procedure. The process cost involved in recapturing and stripping of the solvents is quite significant, and with increasing environmental concerns, the requirements for avoiding release of solvents into the atmosphere have become more strict and therefore more costly.
A typical synthetic process for the synthesis of sulfo-N-hydroxysuccinimide salt is known to follow the following route:
Maleic anhydride: ##STR3## is reacted with furan: ##STR4## to form a Diels-Alder reaction product: ##STR5##
This intermediate product is extremely hazardous and special precautions are required in its handling. Workers must be protectively clothed and may even be required to wear full closure protective gear (e.g., full body suits), including self-contained helmets and at least filters, if not self-contained air supplies. The crystals formed are hazardous to the eyes and are easily propelled and carried by air currents. Even removal of protective garments can be hazardous because of clinging crystalline product which can be put into the air by movement of the clothing. The Diels-Alder product is then reacted with hydroxylamine (e.g., from the hydrochloride salt). ##STR6## This step is done with potassium hydroxide and methanol (precipitating potassium as potassium chloride) producing an N-Hydroxy succinimide adduct: ##STR7##
This product is usually washed in toluene and hexane. The N-hydroxy succinimide adduct is then reacted at the hydroxyl group. This reaction is performed by combining the adduct with phenylchloroformate EQU C.sub.6 H.sub.5 O.sub.2 CC1
(which is a strong lachrymator) in various combinations of triethylamine, dichloromethane, toluene and hexane and sometimes dimethyl formamide to produce the next intermediate product: ##STR8##
This intermediate product is in turn dissolved in a hydrocarbon solvent, e.g., a non-polar hydrocarbon solvent (e.g., decane) and heated to elevated temperature to form the next intermediate by removal of the protecting group. ##STR9##
The temperature is elevated to about 170.degree. C., which is above the flashpoint for decane (46.degree. C.). The literature also shows the use of nitrobenzene as the solvent in this step. The reaction product tends to be a black, tarry product as result of using this commercially difficult step. t-Butyl catechol may be used as an antioxidant in this step.
This last intermediate is then reacted with sodium metabisulfite in ethanol to form the sodium salt of sulfo-N-hydroxysuccinimide, which is precipitated as an amorphous solid from aqueous methanol, isopropanol, and washed with acetone: ##STR10## wherein X.sup.+ is any cation.
This product is produced in about 95-98% purity as an amorphous solid. Even after repeated purification, there is a clear presence of the succinimide counterpart: ##STR11## in the final product. Overall yield of the process from the original maleic anhydride is about 25-28% theoretical, and the complete time of the process is about 50 days. Numerous solvent strips must occur, and a kilogram of product is usually produced in reaction vessels of fifty liters or more.
It therefore can be seen that the entire synthetic route is complex, has toxicity, environmental and hazard concerns throughout, is expensive, and is time consuming. Improved methods of synthesis are clearly desirable.
The methyl diester of sulfosuccinate has been reported in the literature, mainly as a copolymer or coating composition, as shown in FR 2 292 314 (copolymer); JP 87-83257 (corrosion inhibitor); JP 88-307097 (alkali metal alkylsuccinates in coatings and adhesives); and JP 89-84305 (anticlouding compositions of the sodium salt of dimethylsuccinate).
In copending U.S. Pat. application Ser. No. 08/932,851, titled "PREPARATION OF SULFO-N-HYDROXY SUCCINIMIDE SALTS," filed the same day as this application in the names of Marty Wilkes and Martin Bremmer bearing Attorneys Docket Number SLWK 544.005US1, the salt of a sulfonated succinic acid is cyclized (e.g., with a Blanc reaction), then converted to novel sulfonated hydroxamic acids by reaction with hydroxylamine, and the novel hydroxamic acid is then cyclized (e.g., by dehydration) to the sulfo-N-hydroxysuccinimide salt. This is a substantially improved method of synthesis over the prior art, but any other advance over the prior art is also useful.